1. Field of the Invention
The invention relates to a color picture display device comprising:
means for applying a color video signal composed of a gamma-precorrected luminance component and gamma-precorrected chrominance components, PA1 a matrix circuit for deriving gamma-precorrected primary colour signals from the applied gamma-precorrected luminance and chrominance components, PA1 gamma correction means coupled to the matrix circuit for modifying the gamma of the primary color signals, which gamma correction means have a control input for controlling the gamma of the primary color signals in response to a gamma control signal applied to the control input, PA1 means for generating said gamma control signal, PA1 and a color picture display screen coupled to the gamma correction means for displaying the color picture represented by the gamma-corrected color signals. PA1 a sensor device for converting a color picture into a plurality of primary color signals, PA1 gamma correction means coupled to the sensor device for converting the primary color signals into gamma-precorrected color signals, which gamma correction means have a control input for controlling the gamma in response to a gamma control signal applied to the control input, PA1 means for generating said gamma control signal, PA1 a matrix circuit for deriving a gamma-precorrected luminance component and gamma-precorrected chrominance components from the gamma-precorrected color signals, and according to the invention such a camera is characterized by a color correction circuit for modifying, in response to the gamma control signal, the mutual ratio of the primary color signals either before or after the gamma correction means.
2. Description of the Related Art
A device of this type is known, for example, from U.S. Pat. No. 4,489,349.
Conventional color picture display devices have a brightness level control facility with which the user can adjust the brightness of the picture to be displayed. This brightness adjustment is realized by adding an adjustable dc component to the luminance component of the color video signal (or to the primary color signals derived from the luminance component and the chrominance components). However, such a brightness level control has a number of drawbacks:
Not only the grey level values of the video signal but also the peak-white level values are increased when turning up the brightness level. This may lead to unacceptable high white level values so that the video signal must subsequently be limited again to acceptable peak-white level values.
Moreover, the black level of the video signal is raised to a grey level so that the contrast of the displayed picture deteriorates essentially.
Furthermore, an uncolored luminance component is added to the colored parts of the picture. The color saturation is thereby reduced considerably.
A considerable improvement of the picture quality can be obtained by refraining from a brightness level control and by providing instead an adjustable or controllable gamma control which operates on all primary color signals. Such a gamma control can be represented by the formula S.sub.out =S.sup..gamma..sub.in in which S.sub.in denotes the primary color signal applied to the gamma control as an input signal, with S.sub.in =0 for black and S.sub.in =1 for white, and in which S.sub.out represents the gamma-corrected output signal. If the exponent .gamma. is varied, the output signal S.sub.out varies too, but its black level (S.sub.out =0) and the white level (S.sub.out =1) are then independent of .gamma.. It is to be noted that gamma circuits can be used in practice which do not supply an exact exponential gamma but which substantially approximate the function S.sub.out =S.sup..gamma..sub.in, at least for the relevant signal range.
The adjustable gamma in the picture display device forms part of the overall gamma of the transmission system, ranging from the picked-up image in the camera up to and including the displayed image. Conventional gamma values are: 1/2.2 in the camera and 2.8 in the cathode ray tube so that the overall gamma is 2.8/2.2=1.27 without any variable gamma control. With a gamma being adjustable between, for example 1 and 0.40, the overall gamma varies between 1.27 and 2.8*0.40/2.2=0.51.
However, it has been found that a colour error then occurs. This color error becomes manifest as an error in the color saturation and, to a lesser extent, in the color hue.